A semiconductor structure of this type is present both in vertical diodes and vertical transistors and in thyristors, the second, more weakly doped semiconductor zone serving as a drift path which, in the off-state case of the component, takes up the majority of the voltage present between the first and third semiconductor zones.
In diodes, the third semiconductor zone is of the same conduction type as the second semiconductor zone. The second semiconductor zone and the third semiconductor zone are usually n-doped, so that the first semiconductor zone forms the anode and the second semiconductor zone forms the cathode.
In power MOS transistors, a field effect structure is generally present in the region of the first semiconductor zone, which usually lies in the region of the front side of a semiconductor body, said field effect structure comprising a zone of the second conduction type (which is complementary to the doping of the first semiconductor zone), which is arranged in the first semiconductor zone, and a control electrode. The first semiconductor zone forms the so-called body zone of the component, and the complementarily doped zone arranged in the body zone forms the source zone or emitter zone. The control electrode or gate electrode extends, in a manner insulated from the semiconductor zones, from the source or emitter zone as far as the second semiconductor zone, the drift zone. The source zone or the emitter zone and the first semiconductor zone are usually short-circuited, so that a freewheeling diode (body diode) is connected in parallel with the power transistor.
If the power transistor is formed as a MOSFET (Metal Oxide Field Effect Transistor), then the third semiconductor zone is of the same conduction type as the second semiconductor zone or the drift zone and forms the drain zone of the component.
If the power transistor is formed as an IGBT (Insulated Gate Bipolar Transistor) or as a thyristor, then the third semiconductor zone is doped complementarily to the second semiconductor zone and forms the collector zone of the semiconductor component. In thyristors, the first zone is adjoined by a complementarily doped zone.
Diodes, MOSFETs, IGBTs and thyristors of this type are generally known.
EP 0405 200 A1 describes an IGBT of this type, for example, in the drift zone of which the source zone is preceded by a heavily doped zone of the same conduction type as the drift zone, and said heavily doped zone is intended to have the effect that holes injected into the drift zone from the p-doped drain zone do not reach the source zone but rather recombine in said heavily doped zone, which is constructed from a plurality of spaced-apart sections in one embodiment.
All of the components mentioned are subject to the problem that current chopping can occur during the turn-off of the component, that is to say during the transition of the component from a current-conducting to a blocking state. This refers to a process in which the current of the component falls extremely rapidly to very small values. Since the circuitry of such components means that parasitic inductances are unavoidably present, particularly in the leads, and the voltage in these inductances, as is known, is proportional to the derivative of the current, a rapid decrease in the current to very small values effects a high induced voltage that may lead to damage to the component. Furthermore, the appearance of abrupt current changes may be undesirable for specific applications, for example when using a diode as a freewheeling diode in a semiconductor component.
A very rapid fall in the current when the component turns off results from the fact that the second semiconductor zone is initially still flooded by charge carriers which are transported away from the second semiconductor zone, the drift zone, on account of a space charge zone propagating in a manner proceeding from the pn junction between the first and second semiconductor zones. As long as this transporting away of the “stored” charge carriers (plasma charge) lasts, a current which decreases slowly still flows through the connecting lines or to connected loads. As soon as the space charge zone occupies the entire semiconductor body and free charge carriers are no longer present, said current falls with a large temporal gradient to very small values.
In order to avoid this problem, it is known to make the dimensions of the second semiconductor zone as large as possible in the vertical direction of the semiconductor component, so that, during turn-off, charge carriers are subsequently supplied for as long as possible in order to ensure a “soft” turn-off, i.e. a slowest possible decay of the current. What is disadvantageous in this case is that the losses increase since, as the thickness of the drift zone increases, the forward resistance also increases.
DE 102 14 176.2 which has not yet been published, proposes a stop zone formed in sections for the purpose of obtaining a soft turn-off behavior, said stop zone having more heavily doped zones arranged at a distance from one another in the lateral direction of the semiconductor body.